Working principle of Pelton turbine is simple. When a high speed
water jet injected through a nozzle hits buckets of Pelton wheel; it
induces an impulsive force. This force makes the turbine rotate. The
rotating shaft runs a generator and produces electricity.
Fig.1 Pelton wheel derives rotation from impulse force produced by the water jet
In short, Pelton turbine transforms kinetic energy of water jet to rotational energy.
Governing in Pelton Wheel
Demand of power may fluctuate over time. A governing mechanism which
controls position of the spear head meets this requirement. With
lowering power demand the spear head at water inlet nozzle is moved in.
So that water flow rate is reduced. If power demand increases spear
head is moved out this will increase the flow rate. Following figure
illustrates this mechanism. The first position of the spear head
produces a low flow rate, while the second position produces a high flow
rate.
Fig.2 Water flow rate control in Pelton wheel by monitoring position of spear head
So in Pelton turbine synchronization between power demand and power
supply is met by controlling the water flow rate. The same technique is
used in other types hydroelectric turbines. If the power supply is more
than the demand, then the turbine will run over speed otherwise in under
speed. But such a governing mechanism in turn will balance the power
supply and demand and will make sure that the turbine rotates at a
constant specified RPM. This speed should also conforms to the
power supply frequency. So this mechanism acts as a speed governing
mechanism of Pelton wheel.
Number of Buckets in Pelton Wheel
One of the most important parameter of Pelton turbine design is
number of buckets on the disk. If number of buckets is inadequate, this
will result in loss in water jet. That means when one bucket departs
from the water jet next bucket may not get engaged with the jet. This
will result in loss in water jet for a small time duration, thus sudden
drop in turbine efficiency. Following figure illustrates what happens
when the number of buckets are lowered. With lowering number of buckets
at some point of operation, complete water jet might be lost (3rd figure). So there should be an appropriate number of buckets, which will make sure that no water is lost (1st figure).
Fig.3 Effect of number of buckets on water-bucket interaction
Pelton Bucket - Design & Features
Most vital component of Pelton wheel is its bucket. Buckets are
casted as single solid piece, in order to avoid fatigue failure. You can
note that force acting on the turbine bucket is not constant with time.
If you follow one particular bucket, it will have high force for a
small time duration (at the time of jet impingement) after that a larger
idle period where no jet interaction takes place. So the force acting
on the bucket is also not constant. It varies with the time but it is
having a cyclic nature. If bucket were made using pieces by welding
attachment such cyclic fore will easily lead to premature fatigue
failure.
Fig.4 Different views of Pelton bucket
Water jet is split into 2 equal components with help of a splitter.
The special shape of bucket makes the jet turn almost 180 degree. This
produces an impulsive force on bucket. Force so produced can easily be
derived from Newton’s 2nd law of motion. Blade outlet angle close to
180 degree is usually used in order to maximize impulsive force.
A cut is provided on bottom portion of buckets. This makes sure
that water jet will not get interfered by other incoming buckets.
Pelton – An Impulse Turbine
Since the water jet is always open to atmosphere, inlet and exit
pressure of water jet will be same and will be same as atmospheric
pressure. However absolute velocity of fluid will have huge drop from
inlet to exit of bucket. This kinetic energy drop is the maximum energy
the bucket can absorb.
So it is clear that Pelton turbine gains mechanical energy purely due
to change in kinetic energy of jet, not due to pressure energy change.
Which means Pelton turbine is a pure impulse machine.
Fig.5 Pressure and velocity variation across Pelton bucket
Impulse force produced by water jet is high when jet is having high
velocity. Water stored at high altitude can easily produce high jet
velocity. This is the reason why Pelton turbine is most suitable for
operation, when water is stored at high altitude.
You can easily understand why there is a nozzle fitted at water jet
injection portion. Nozzle will increase velocity of jet further, thus
will aid in effective production of impulse force.
Extracting Maximum Power from Water Jet
Pelton turbine design is always aimed at extracting maximum power
from water jet, or maximizing efficiency. Power extracted by the bucket,
P is product of jet impulse force and bucket velocity.
So power extraction is maximum when product of impulsive force and
bucket velocity is maximum. Let's consider 2 different operating
conditions.
Buckets are Held Stationary
If Pelton wheel buckets are held stationary, there will be a huge
impulse force produced. But power extraction will be zero since buckets
are not moving.
Bucket Speed Same as Jest Speed
If buckets are moving with same speed of jet, water jet won't be
able to hit the bucket. This will lead to zero impulse force. Again
power extraction will be zero.
Fig.6 Relative magnitude of bucket and jet velocity is important in power extraction from fluid
In short, power extraction is zero both at zero bucket speed and when
bucket speed is same as jet speed. So with respect to jet to bucket
speed ratio, power extraction will vary with as shown below.
Fig.7 This graph shows how power extraction from fluid varies with respect to bucket to jet velocity ratio
It is clear from the above graph that optimum power extraction happens
in between. It can be shown using Euler's turbo machinery equation that
maximum power extraction happens when bucket speed is half the jet
velocity. So it is always desirable to operate Pelton wheel at this
condition. Pelton turbines can give efficiency as high as 90 %, at
optimum working conditions.
